Field of the Invention
The invention relates to corneal reshaping and more particularly to electrodes utilizing radiofrequency electrical current to heat and thereby induce reshaping of the cornea in animals including humans.
Extreme cases of refractive error, such as those caused by keratoconus, are frequently not correctable by the addition of external refraction. Corneal transplant is the usual remedy. Recent alternatives have been suggested which include modification of the corneal shape by thermal methods which rely upon dramatic shrinkage of corneal collagen in about the 55.degree. to 65.degree. Centigrade (C) range. The use of these thermal methods has been limited by damage to the epithelium and Bowman's membrane, and the temporary nature of the change effected. Each of these problems appear to be related to the thermal dose profile within the cornea which is generated by the conductive heating apparatus. Temperatures reached in the epithelium are relatively high while temperatures reached in the deeper stromal collagen are below the critical shrinkage temperature needed. A wide variability in treatment results exists which is probably caused by unintentional variation in individual techniques in administering thermal treatments, particularly in the length of time that heat is applied.
The use of heat to alter the shape of the cornea is generally referred to as thermokeratoplasty (TKP). A device called a thermokeratophore has been used by several medical researchers. The essential part of the device is a cylindrical metal probe that is electrically preheated to some desired temperature; usually in the range of between about 90.degree. and 130.degree. C. The probe is then applied directly to the cornea for one to two seconds. The cornea is heated by thermal conduction from the metal probe; surface temperature at the cornea is significantly higher than deeper (stromal) temperature. Measurements of the temperature profile achieved in rabbit corneas are described by E. L. Shaw and A. R. Gasset in "Thermokeratoplasty (TKP) Temperature Profile," J. Invest. Ophthalmol. 13, No. 3, 181-186 (1974). For example, a two second application of a 90.degree. C. probe produced epithelial and endothelial temperatures of about 69.degree. and 53.degree. C. respectively. Central corneal stroma would have been treated to about 60-62.degree. C., probably high enough for shrinkage to occur. The rabbit cornea is relatively thin, being only about 400 microns thick. Shrinkage down to depths of about 200 microns might be sufficient for a thin cornea such as that of the rabbit, buut would probably only cause minimal or transient change in shape when corneal thickness exceeds 400 microns. Human corneal thickness typically varies from 580 microns centrally to 1000 microns at the periphery. The reason the invention uses radiofrequency electric current for heating is that it is essential to penetrate at least a few hundred microns in depth to effect corneal changes in larger mammals, such as man.